Move fastlog2() to WTF/MathExtras.h so it can be used from multiple projects.

Rubberstamped by Geoff Garen.

Source/WebCore:

* loader/cache/MemoryCache.cpp:
(WebCore::MemoryCache::lruListFor): Remove the inline fastlog2 and use WTF::fastlog2.

Source/WTF:

* wtf/MathExtras.h:
(WTF::fastLog2):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@143687 268f45cc-cd09-0410-ab3c-d52691b4dbfc